1. Field of the Invention
This invention relates to a liquid droplet ejecting apparatus and a method of driving a liquid droplet ejecting head which ejects liquid droplets through nozzles.
2. Description of Related Arts
An ink jet recording head (hereinafter simply referred to as a recording head) is used to record images with very small ink droplets. A liquid droplet ejecting head as the ink jet recording head which ejects liquid droplets through nozzles jets out liquid droplets onto a recording medium such as a sheet of recording paper by giving a pressure to a pressure generation chamber.
Various kinds of pressurizing devices have been available to give pressures to the pressure generation chambers. Among the pressurizing devices, we briefly explain a recording head which is disclosed by Patent Document 1 referring to FIGS. 12(a) and (b). The partition walls of the pressure generation chamber are made of piezoelectric elements and deformed to eject ink droplets through nozzles.
As shown in FIGS. 12(a) and (b), the above shear-mode recording head 600 contains bottom wall 601, ceiling wall 602 and shear-mode actuator walls 603 therebetween. Each actuator wall 603 contains lower wall 607 which is bonded to bottom wall 601 and polarized in the arrow direction 611 and upper wall 605 which is bonded to ceiling wall 602 and polarized in the arrow direction 609. A pair of actuator walls 603 forms ink flow channel 613 which work as a pressure generation chamber therebetween. Nearby actuator walls 603 of two pairs of adjacent actuator walls form space 615 which is narrower than ink flow channel 613. This space 615 is a dummy channel and does not eject ink. In other words, this head is a so-called dummy channel type head.
Nozzles plate 617 with nozzle 618 is firmly fixed to one end of each ink flow channel 613. Each surface of actuator wall 603 has a metal layer of electrode 619 or 621. Each of electrodes 619 and 621 is covered with an insulating layer (not shown in drawings) to insulate from ink. Electrodes 621 facing to space 615 is connected to ground 623. Electrode 619 provided in ink flow channel 613 is connected to silicone chip 625 which works as an actuator driving circuit.
Meanwhile, for fast recording of ink jet images, it is necessary to drive recording head 600 at a high-frequency and eject ink droplets at shorter cycles. Specifically, to accomplish high-frequency greyscale driving, it is necessary to eject an ink droplet and the next droplet promptly and stably through the identical nozzle.
For this purpose, Patent Document 1 discloses a method of applying a cancellation pulse after ejecting an ink ejection pulse to reduce a pressure change in ink flow channel 613 which is a pressure generation chamber.
In other words, this method applies a cancellation pulse to generate a pressure wave whose phase is opposite to a pressure change in ink flow channel 613 a preset time later after an ink droplet is ejected. As shown in FIG. 13, a cancellation pulses D (pulse width AL) whose phase is opposite to that of ejection pulse C is applied to electrode 619 of ink flow channel 613 time lapse AL later after ejection pulse C falls. Here, AL is ½ of an acoustic resonance period of the pressure generation chamber.
The voltage value of the cancellation pulse is determined according to the amplitude of a pressure change to cancel the change (for example, 0.6 time of the ejection pulse voltage). When receiving this cancellation pulse, actuator wall 603 deforms in a direction opposite to deformation of the actuator wall at the time of ink ejection. This eliminates a change in ink ejection velocity when the frequency of drive pulses is changed and consequently improves the printout quality. This also enables quick stable ejection of a succeeding ink droplet after ejection of an ink droplet through an identical nozzle. Consequently, the recording head can be driven at a high frequency and eject ink droplets at a shorter cycle.
[Patent Document 1] Japanese Non-Examined Patent Publication 2003-276200
This method like a conventional driving method can cancel a pressure wave in a pressure generation chamber by applying a cancellation pulse. Thereby, the recording head can greatly attenuate the meniscus vibration due to a pressure wave in the pressure generation chamber and start ejection of the next ink droplet.
FIG. 5(b) shows a behavior of an ink meniscus in a nozzle and how a liquid droplet is ejected in a conventional driving method. FIG. 5(b) shows nozzle 23, ink pillar 102, ink droplet 101, main droplet 11, satellite droplet 12, and meniscus M. FIGS. 5(a), (b) will be explained in detail later.
From present inventors' stock of information, the following is found:
When a cancellation pulse is applied to cancel a pressure wave as disclosed in the above prior art as shown in FIG. 5(b), ink pillar 102 still has its root in meniscus M. (See FIG. 5(b)-(3) and (5).) As ink pillar 102 extends away from nozzle 23, ink pillar 102 is pulled by the moving energy of the main droplet and becomes longer since only the surface tension of the ink works to cut off the ink pillar 102. Then, the separated ink droplet 101 also becomes longer and consequently the velocity difference between the top and tail of the ink droplet becomes greater. When the flying force of the liquid pillar overcomes the surface tension which works to cut off the liquid pillar, ink droplet 101 is apt to part into main droplet 11 which has a preset volume and a preset ejection velocity and satellite droplets 12 each of which has smaller volume and ejection velocity than those of the main droplet volume. Consequently, the number of satellite droplets 12 increases. (See FIG. 5(b)-(9) and (10).)
This phenomenon appears more eminently when ink of a low surface tension or high viscosity is ejected.
Such satellite droplets 12 will land off the landing position of main droplet 11, which causes the deterioration of image qualities.
In the above ink jet recording head, any crud in the vicinity of a nozzle on the outer side of the nozzle forming member will interfere with the ejection of ink, causing the ejected ink to change its flight, to be dragged in and deposited near the nozzle. Further, it frequently occurs that satellite droplets 12 float around the recording head and are deposited near the nozzles on the outer side of the nozzle forming member. This also causes the above problem.
Besides, when satellite droplets 12 increase its number, the ink mist increases in the recording apparatus. The ink mist will contaminate the inside of the apparatus and, in an extreme case, the ink mist deposits on electric contacts and causes malfunction of the apparatus.
This invention has been made in view of the above problems and an object of this invention is to provide a liquid droplet ejecting apparatus which can be driven at a high-frequency, reduce the number of satellite droplets, and ejects liquid droplets steadily, and a method of driving a liquid droplet ejecting head to accomplish the object.